The present invention relates to a process for the manufacture of unsaturated polyester resins exhibiting linear maturing with magnesia. Its subject is more particularly a process for the manufacture of polyester resins based on ortho-phthalic anhydride, which exhibits linear maturing with magnesia whatever the proportion of magnesia employed.
Polyester resins are known products which are employed in various applications, in particular for the manufacture of preimpregnated materials. The latter materials are made from reinforcing fibres and from unsaturated polyester resins which act as impregnating compositions. In order to produce a material which can be handled and stored before moulding, it is known to add a thickener to the impregnating composition before impregnating the reinforcing fibres.
These impregnating compositions have hitherto been prepared by reacting at least one polyol with one or more .alpha.,.beta.-unsaturated polycarboxylic acids or anhydrides and, if appropriate, one or more saturated polycarboxylic acids or anhydrides. The polycondensate prepared in this way is then dissolved in a crosslinking vinyl monomer. As polyols which are employed, it is possible to mention ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, neopentyl glycol, butylene glycol, dipropyleneglycerol, trimethylolpropane, pentaerythritol, trimethylpentanediol, trimethylpropanediol 1,6-hexamethylenediol, and hydrogenated bisphenol A and its derivatives. The .alpha.,.beta.-unsaturated polycarboxylic acids and anhydrides are represented especially by maleic and itaconic acids and anhydrides and fumaric and mesaconic acids. As saturated polycarboxylic acids and anhydrides it is possible to mention succinic, methylsuccinic, adipic, sebacic, ortho-pththalic, tetrachlorophthalic, hexachloroendomethylenetetrahydrophthalic and trimellitic acids and anhydrides and isophthalic acid. The crosslinking vinyl monomer employed is chosen from styrene, .alpha.-methylstyrene, chlorinated styrene derivatives, vinyltoluene, divinylbenzene, allyl phthalate, triallyl cyanurate, and the lower alkyl acrylates and methacrylates.
In actual fact, among all these monomers mentioned above, the impregnating compositions are in most cases manufactured by reacting, in a single step, a polyol mixture consisting of propylene glycol or ethylene glycol and diethylene glycol with a mixture of maleic anhydride and ortho-phthalic anhydride. The polycondensate thus produced is dissolved in styrene containing small amounts of an inhibitor such as hydroquinone or benzoquinone (50 to 150 ppm).
The preimpregnated materials are compounds consisting chiefly of a mixture of unsaturated polyester resin, filler, colorant, release agent, glass fibres, crosslinking agent and thickener. This composition, initially liquid, is not sufficiently capable of being handled to be moulded as such. Before the moulding it must be matured to raise its viscosity. One of the known maturing processes consists in manufacturing a sandwich material consisting of the preimpregnated composition, which must retain its polymerization and crosslinking capacity, enclosed between two polyethylene films. The thickener causes an increase in viscosity which makes the sandwich material capable of being handled after a certain time. Typical thickeners include magnesia.
After maturing and, in the case of that mentioned above, removal of the polyethylene films, the preimpregnated materials can be moulded according to the usual hot compression-moulding processes, the crosslinking being produced by decomposition of the crosslinking agent which is usually an organic peroxide.
The thickener added for the maturing causes an increase in the viscosity of the prepregs which must be such that the viscosity is sufficient to permit moulding. When the viscosity is inadequate, major leakages of materials are observed on the mould and, when the viscosity is toc high, the prepregs flow poorly in the mould, causing faulty appearance and poor mechanical properties of the moulded part. More precisely, it is found that when the proportion of magnesia is varied in the case of the known unsaturated resins:
at a given time, no maturing below a certain proportion of magnesia, the maturing appearing starting at a certain threshold,
after a certain storage time, a loss in plasticity whatever the proportion of magnesia employed, causing non linear variations in maturing.